Not Applicable.
1. Field of Invention
The present invention relates to an apparatus for sensing variations in an inductive field and a method for deploying the inductance sensing apparatus. More specifically, the present invention relates to geometries and configurations for inductive sensors that are capable of producing more information than conventional inductive sensors. Additionally, the present invention relates to methods and configurations for deploying inductive sensors in locations where conventional inductive sensors are too expensive, invasive, impracticable, or inconvenient.
2. Description of the Related Art
The use of inductive wire-loop sensors with oscillator-based vehicle detectors is known to those skilled in the art. The conventional configuration, which is in common use throughout the United States, is a loop that is oriented substantially parallel to the roadway surface. The loop is a wire that is laid in a series of channels roughly defining a rectangle. Typically, there are eight cuts that make up the rectangular configuration. These include the four sides and four angular cuts, each angular cut joining two adjacent orthogonal sides. A flexible wire is placed in the cuts and the cuts are sealed. It is also known to place a wire-loop in a circular cut.
The dimensions of conventional wire-loop sensors are selected to maximize the coverage area and detect the widest variety of vehicle types while minimizing interference from electromagnetic noise and crosstalk. Generally, the signal strength of the variations in the inductive field is strongest when a vehicle passes over the entire roadway loop. Increasing the area of the roadway loop so that a vehicle passes only over part of the loop decreases the signal strength and increases the susceptibility to electromagnetic noise. For roadway loops having widely spaced parallel legs, the resulting poor signal-to-noise ratio makes it difficult to reliably detect the presence of differing classes of vehicles using the same roadway loop. Accordingly, conventional roadway loops are dimensioned so as to detect a typical vehicle with all four legs of the loop simultaneously. This results in roadway loops that are necessarily narrower than the width of a single standard twelve-foot traffic lane.
Additionally, conventional free-running oscillators used to drive the conventional roadway loops require those roadway loops in adjacent lanes to be separated by a fair distance to minimize crosstalk. For a multi-lane roadway having standard twelve-foot traffic lanes, the conventional roadway loops typically have a width of approximately six feet and are centered within the lane to provide maximum separation from roadway loops in adjacent lanes. As taught by current usage, the four legs of the roadway loop generally follow the shape of a typical vehicle as oriented in the flow of traffic along the roadway where the roadway loop is disposed. Although a conventional roadway loop used in a multi-lane roadway may have a lead line extending outside of the traffic lane to connect the roadway loop to a controller, the inductive field generating legs of the conventional roadway loop are not known to extend across the entire width of a roadway.
As previously discussed, the installation of each inductive sensor in an existing roadway requires cutting the roadway surface to receive the inductive sensor, together with the additional cuts necessary to connect the inductive sensor to the controller. The conventional method of installing a vehicle detection system includes placing a series of inductive sensors in the roadway and connecting them to distantly located controllers through trenches dug beside the roadway. A major portion of the cost of installing a vehicle detection system that links disparate sections of highway, as in the case of a traffic flow monitoring system along a freeway, is associated with the trenching operation in the form of insurance against cutting underground communication or power lines due to monetary penalties for interruption of service. Accordingly, the cost is artificially inflated and does not bear a reasonable relation to the actual effort and expense incurred for the acquisition and installation of the vehicle detection system.
An inductive sensor capable of providing information as to the lateral offset of a vehicle within a traffic lane is disclosed. The inductive sensor is generally configured such that the angular offset between the generally horizontal plane, which represents the roadway surface, and the plane defined by the longitudinal legs of the inductive sensor varies with the length. In one embodiment, the inductive sensor includes two wire-loops with each wire-loop having an orientation that varies along the length of the sensor. The wire-loops are displaced from each other by an angular offset.
In an alternate embodiment, the inductive sensor includes a pair of generally coplanar wire-loops. The outside legs of the co-planar wire-loops generally form a quadrilateral. The quadrilateral defines a longitudinal axis that is bisected at the midpoint by a pair of abutting inside legs, one from each wire-loop. The inductive field for the two wire-loops is balanced under normal conditions. However, in the presence of a vehicle, the inductive field of the wire-loops becomes unbalanced allowing the lateral offset of the vehicle within the roadway to be determined. The inductive sensor is disposed either substantially parallel or substantially perpendicular to the roadway surface. One embodiment includes two substantially parallel, substantially concentric inductive sensors, each inductive sensor including two wire-loops. When in the substantially concentric orientation, one inductive sensor is placed closer to the roadway surface than the other inductive sensor so that the inductive field is adapted to detect wheel spikes. In the substantially perpendicular orientation, the two wire-loops of the inductive sensor are typically placed in an over-under arrangement.
The composite of the measured inductance of a vehicle obtained using the inductive sensor remains consistent for a given vehicle. However, the measured inductance from each of the wire-loops varies depending upon the lateral offset of the vehicle within the traffic lane.
The ability to detect the lateral offset of a vehicle within a roadway allows the inductive sensor of the present invention to be installed without having to be matched to the final position of the traffic lanes. It allows for self-calibration of the system that relaxes the need for costly and time-consuming installations.
Installation of an inductive sensor during the construction of a new roadway and the resurfacing or repair of an existing roadway can be accomplished by simply embedding the inductive sensor in the roadway during the paving process at a reduced cost and a reduced inconvenience. However, it is not always considered, currently desired, or budgeted to install a vehicle detection system when road repair or construction occurs. By using a vehicle detection installation system including a conduit along the length of the roadway that provides access to a series of detectors, spaced at a desired interval, the decision to install a complete vehicle detection system can be delayed without excessive additional cost.
Generally, the conduit carries power and communications and is adapted to allow inductive detectors to be connected to the network. This allows the detectors to be spaced more closely than the typical one-third to one-half mile spacing because of ready access to power and communications. An access port provides access to the interior of the conduit, to allow for installation, maintenance or repair of the vehicle detection system hardware.
In another embodiment, the inductive sensor requires little or no cutting of the roadway surface, i.e., nondestructive, so as to leave the structural integrity of the roadway intact and to reduce the cost of installation. This is useful on roadways where cutting is either undesirable or prohibited such as on bridges. It is further useful in areas where a large area of detection is desired or where lanes of travel are not clearly defined. Finally, such an inductive sensor is useful where a temporary installation is needed. The inductive sensor is formed using a conductive material painted on, or otherwise adhered to the roadway surface, or filled into shallow grooves.